Catalyst precursors, catalysts and methods of producing same

ABSTRACT

A catalyst precursor comprising (A) a microporous support; (B) a non-noble metal precursor; and (C) a pore-filler, wherein the micropores of the microporous support are filled with the pore-filler and the non-noble metal precursor so that the micropore surface area of the catalyst precursor is substantially smaller than the micropore surface area of the support when the pore-filler and the non-noble metal precursor are absent is provided. Also, a catalyst comprising the above catalyst precursor, wherein the catalyst precursor has been pyrolysed so that the micropore surface area of the catalyst is substantially larger than the micropore surface area of catalyst precursor, with the proviso that the pyrolysis is performed in the presence of a gas that is a nitrogen precursor when the microporous support, the non-noble metal precursor and the pore-filler are not nitrogen precursors is also provided. Methods of producing the catalyst precursor and the catalyst are provided.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. Ser. No.13/127,744, filed Jul. 19, 2011, now U.S. Pat. No. 8,580,704, which is anational stage application under 35 USC §371 of the internationalapplication number PCT/CA2009/001365 filed Oct. 2, 2009, which claimspriority on U.S. provisional application Ser. No. 61/112,844 filed onNov. 10, 2008. The contents of these priority applications areincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to catalyst precursors, catalysts, andmethods of producing these catalyst precursors and catalysts. Morespecifically, the present invention is concerned with non-noble metalcatalysts. Such materials can be used in oxygen reduction reactions infuel cells, including acid or alkaline polymer electrolyte membrane fuelcells and metal-air batteries.

BACKGROUND OF THE INVENTION

Clean, efficient and versatile, H₂—O₂ (air) polymer electrolyte membranefuel cells (PEMFCs) are seen as worthy alternatives to a wide range ofconventional power generation devices such as internal combustionengines, batteries and diesel-fuelled back-up power systems. PEMFCsgenerate electricity via two electrochemical reactions that involve theoxidation of hydrogen at the anode (2H₂→4H⁺+4e⁻) and the reduction ofoxygen at the cathode (O₂+4H⁺+4e⁻→2 H₂O), thus producing only water andheat. Due to the rather low operating temperature of PEMFCs (ca. 80°C.), catalysts play an essential role in boosting the reaction kineticsto produce the desired high power densities.

Today, the only viable electrocatalysts used in PEMFCs areplatinum-based. Platinum is considered to be a “noble” metal, such asgold, for example. In PEMFCs, 90% of the platinum is needed at thecathode due to the sluggishness of the oxygen reduction reaction (ORR)compared to the fast hydrogen oxidation reaction at the anode. Despiteimproved platinum performances, the increasingly prohibitive cost ofplatinum remains a major obstacle for the commercial viability ofPEMFCs.

In addition, the production of platinum in the natural world is ratherlimited. On the other hand, there are estimations that the demand forplatinum as electrode catalyst will increase significantly as the demandfor electric cars with fuel cells increases. Therefore, there is a fearof a still further rise in platinum prices. Accordingly, electrodecatalysts which can be formed without using noble metals, such asplatinum, are desirable.

Research activity into non-noble or non-precious metal catalysts (NPMC)for the ORR has grown considerably since 1964 when Jasinski observedthat cobalt phthalocyanine catalyzed the ORR (REFERENCE 1). Suchcatalysts were first obtained by adsorbing Fe—N₄ or Co—N₄ macrocycles ona carbon support and pyrolysing the resulting material in an inertatmosphere (REFERENCE 2). Since, NPMC research using metal-N₄macrocycles has continued (REFERENCE 3-5).

A breakthrough was then achieved when it was revealed that theseoften-expensive macrocycles could, instead, be substituted by individualN and Co precursors (REFERENCE 6). This approach was followed by severalgroups (REFERENCE 2, 7, 8-17).

One approach in the synthesis of NPMCs for ORR has been to use NH₃ as anitrogen precursor. The catalysts are obtained by wet impregnation of acarbon black with an iron precursor like iron^(II) acetate (FeAc),followed by a heat treatment, i.e. pyrolysis, in NH₃. Herein, suchelectrocatalysts will be referred to as Fe/N/C catalysts. Duringpyrolysis at temperatures ≧800° C., NH₃ partly gasifies the carbonsupport, resulting in a mass loss that depends on the duration of theheat treatment (REFERENCE 18). The disordered domains of the carbonsupport are preferentially gasified (REFERENCE 19-21). As a result,micropores are created in the carbon black particles. The mass loss(30-50 wt %) at which maximum activity is reached corresponds to thelargest microporous surface area of the etched carbon, suggesting thatthese micropores (size ≦2 nm) host the catalytic sites (REFERENCE 19).In addition, the reaction of NH₃ with the disordered carbon domains alsoproduces the N-bearing functionalities needed to bind iron cations tothe carbon support (REFERENCE 22-23).

Hence it has been proposed that each Fe/N/C catalytic site comprises aniron cation coordinated by four pyridinic functionalities attached tothe edges of two graphene planes, each belonging to adjacentcrystallites on either side of a slit pore in the carbon support(REFERENCE 19, 23). Thus, four factors have been identified asrequirements for producing active Fe-based catalysts for ORR: (i)disordered carbon content in the catalyst precursor (REFERENCE 18); (ii)iron; (iii) surface nitrogen and (iv) micropores in the catalyst.Disordered carbon allows for the formation of micropores and nitrogenenrichment during pyrolysis in NH₃, Fe and N are essential because theyform an integral part of the catalytic site (REFERENCE 2), whilemicropores are required to host the catalytic site (REFERENCE 19-21).

For NPMCs, it is meaningful to speak in terms of volumetric activity forORR. Conversion from A·g⁻¹ _(NPMC) to A·cm⁻³ _(electrode) is describedbelow. The volumetric activity is the product of the catalytic sitedensity and the activity of a single site. The latter varies withvoltage and is an intrinsic property of the catalytic site. Therefore,if the site is unchanged, increased volumetric activity can only beachieved by increasing the site density.

The volumetric catalytic activity of a catalyst may be marginallyimproved by increasing the Fe content. However, the inventors previouslyfound increased activity only up to a presence of ca. 0.2 wt % Fe,beyond which the activity levels off and eventually decreases (REFERENCE24). Therefore, a nominal Fe concentration of 0.2 wt % was at that timechosen for impregnation onto pristine non-porous carbon blacks.

Furthermore, when catalysts were prepared using the impregnation methodon non-porous carbon black and pyrolysed in pure NH₃, the microporesurface area of the resulting catalysts was shown to govern thecatalytic activity because the nitrogen and iron content were usuallynon-limiting (REFERENCE 19).

FeAc was therefore impregnated onto highly microporous carbon supportsfollowed by pyrolysis in NH₃. Surprisingly, this did not improve theactivity as compared to catalysts made with non-porous carbon supports.Instead, it was concluded that only the micropores created duringpyrolysis in NH₃ may host catalytic sites (REFERENCE 25). The inventorsthus found that the micropores in the as-received microporous carbonblacks do not bear the surface nitrogen necessary to form catalyticsites. Furthermore, since these carbon blacks have little disorderedcarbon content, surface nitrogen is difficult to add during pyrolysis inNH₃.

REFERENCE 26 gives conditions for measuring the volumetric activity ofNPMCs in fuel cells.

REFERENCES 18 and 22 disclose the use of different carbon blacks andactivated carbon in catalysts of the prior art.

REFERENCES 27 and 28 disclose the use of a carbon pretreated to addnitrogen and/or carbon with a nitrogen-containing molecule with ironacetate in catalysts of the prior art.

REFERENCE 29 discloses the use of iron, cobalt, chromium and manganesein catalysts of the prior art.

REFERENCES 30-38, 9, 39-46, discloses the use of at least the followingnon-noble metal precursors in catalysts of the prior art: cobaltporphyrin (Co tetramethoxyphenylporphyrin (TMPP)); iron acetate, Fetetramethoxyphenylporphyrin (TMPP) on pyrolysedperylene-tetracarboxylic-dianhydride (PTCDA); Fe phthalocyanines; Fe andCo tetraphenylporphyrin; Co phthalocyanines; Mo tetraphenylporphyrin;metal/poly-o-phenylenediamine on carbon black; metal porphyrin;molybdenum nitride; cobalt ethylene diamine; hexacyanometallates;pyrrol, polyacrylonitrile and cobalt; cobalt tetraazaannulene; andcobalt organic complexes.

REFERENCES 29, 56 and 57 report the successful use of Fe^(II) acetate,cobalt acetate, copper acetate, chromium acetate, manganese acetate,nickel acetate, and ferrocene in prior art catalysts.

The present inventors and other authors have successfully used Fe^(II)acetylacetonate, Fe^(II) sulfate, Fe^(III) chloride, Fe^(III) nitrate,Fe^(II) oxalate, Fe^(III) citrate, ChloroFetetramethoxyphenylporphyrine, cobalt phthalocyanine, ironphthalocyanine, cobalt tetra-aza-annulene in catalysts of the prior art.

The present inventors have successfully used all the compounds betweenparenthesis and at least one compound of each family bellow in non-noblecatalysts of the prior art: Phenanthroline (1,10-phenanthroline,Bathophenanthroline disulfonic acid disodium salt hydrate, 4,7-Diphenyland 5,6-dimethyl phenanthroline, 4-aminophenanthroline); Phthalocyanine;Porphyrine; Phthalonitrile (4-Amino-phthalonitrile); Melamine;Hexaazatriphenylene; Tetracarbonitrile;Benzene-1,2,4,5-tetracarbonitrile; amino-acids; Polypyrrole;Polyaniline, Bismark Brown; and Bathocuproine(2,9Dimethyl-4,7-diphenyl-1,10-phenantroline). These results wereunpublished before the present and were thus not part of the prior artavailable to the skilled person.

There remains a need for improved NPMCs to replace the Pt-basedelectrocatalysts used in PEMFCs.

The present description refers to a number of documents, the content ofwhich is herein incorporated by reference in their entirety.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a catalystprecursor comprising: (A) a microporous support; (B) a non-noble metalprecursor; and (C) a pore-filler, wherein the micropores of themicroporous support are filled with the pore-filler and the non-noblemetal precursor so that the micropore surface area of the catalystprecursor is substantially smaller than the micropore surface area ofthe support when the pore-filler and the non-noble metal precursor areabsent.

There is also provided a method for producing a catalyst precursor, themethod comprising: (A) providing a microporous support; a non-noblemetal precursor; and a pore-filler; and (B) filling the micropores ofthe microporous support with the pore-filler and the non-noble metalprecursor so that the micropore surface area of the catalyst precursoris substantially smaller than the micropore surface area of the supportwhen the pore-filler and the non-noble metal precursor are absent.

In embodiments of the above catalyst precursor and method of producingsame, at least one of the microporous support, the non-noble metalprecursor, or the pore-filler is a nitrogen precursor. As noted below,if none of the microporous support, the non-noble metal precursor, orthe pore-filler is a nitrogen precursor, then the pyrolysis gas withwhich the catalyst precursor will be treated should be a nitrogenprecursor.

In embodiments of the above catalyst precursor and method of producingsame, the microporous support is highly microporous. In embodiments, themicroporous support is carbon-based. In embodiments, the microporoussupport is carbon black or activated carbon.

In embodiments of the above catalyst precursor and method of producingsame, the non-noble metal precursor is a precursor of iron or cobalt. Inspecific embodiments, the non-noble metal precursor is a precursor ofiron. In embodiments, the catalyst precursor has an iron loading ofabout 0.2 wt % or more based on the total weight of the catalystprecursor. In more specific embodiments, the catalyst precursor has aniron loading of about 1 wt % based on the total weight of the catalystprecursor.

In specific embodiments, the non-noble metal precursor is a salt of anon-noble metal or an organometallic complex of a non-noble metal. Inmore specific embodiments, the non-noble metal precursor is Fe^(II)acetate.

In embodiments of the above catalyst precursor and method of producingsame, the pore-filler comprises a polycyclic structure. In specificembodiments, the pore-filler is perylene-tetracarboxylic-dianhydride,1,10-phenanthroline, perylene tetracarboxylic-diimide, orpolyacrylonitrile.

In embodiments of the above catalyst precursor and method of producingsame, the non-noble metal precursor and the pore-filler are the samemolecule.

In embodiments of the above catalyst precursor and method of producingsame, the micropore surface area of the catalyst precursor is at mostabout 75%, 65%, 55%, 45%, 35%, 25% or 15% of the micropore surface areaof the support when the pore-filler and the non-noble metal precursorare absent. In specific embodiments of the above catalyst precursor andmethod, the micropore surface area of the catalyst precursor is at mostabout 10% of the micropore surface area of the support when thepore-filler and the non-noble metal precursor are absent.

In embodiments of the above catalyst precursor and method of producingsame, the pore-filler/microporous support mass ratio is 50:50.

In embodiments of the above catalyst precursor and method of producingsame, the micropores of the microporous support are filled with thepore-filler and the non-noble metal precursor by ballmilling or resonantacoustic mixing with or without a grinding medium. In specificembodiments, the ballmilling is planetary ballmilling.

For certainty, in embodiments of the above method for producing acatalyst precursor, the catalyst precursor is as described above.

There is also provided a catalyst comprising the above-describedcatalyst precursor, wherein the catalyst precursor has been pyrolysed sothat the micropore surface area of the catalyst is substantially largerthan the micropore surface area of catalyst precursor, with the provisothat the pyrolysis is performed in the presence of a gas that is anitrogen precursor when the microporous support, the non-noble metalprecursor and the pore-filler are not nitrogen precursors.

Finally, there is also provided a method of producing a catalyst, themethod comprising (A) providing a catalyst precursor comprising amicroporous support; a non-noble metal precursor; and a pore-filler,wherein the micropores of the microporous support are filled with thepore-filler and the non-noble metal precursor so that the microporesurface area of the catalyst precursor is substantially smaller than themicropore surface area of the support when the pore-filler and thenon-noble metal precursor are absent; and (B) pyrolyzing the catalystprecursor so that the micropore surface area of the catalyst issubstantially larger than the micropore surface area of catalystprecursor, with the proviso that the pyrolysis is performed in thepresence of a gas that is a nitrogen precursor when the microporoussupport, the non-noble metal precursor and the pore-filler are notnitrogen precursors.

In embodiments of the above catalyst and method for producing same, themicropore surface area of the catalyst is at least half (50%), 60%, 70%,75% or 80% of the micropore surface area of the support when thepore-filler and the non-noble metal precursor are absent. In specificembodiments, the micropore surface area of the catalyst is at leastthree-quarters (75%) of the micropore surface area of the support whenthe pore-filler and the non-noble metal precursor are absent.

In embodiments of the above catalyst and method for producing same, themass loss during pyrolysis is about equal to the pore-filler loading inthe catalyst precursor (in weight % based on the total weight on thecatalyst precursor).

In embodiments of the above catalyst and method for producing same, thecatalyst has a nitrogen content of about 0.5 wt % or more based on thetotal weight of the catalyst.

In embodiments of the above catalyst and method for producing same, thecatalyst is an oxygen reduction catalyst, a catalyst for thedisproportionation of hydrogen peroxide or a catalyst for the reductionof CO₂. In specific embodiments, the catalyst is an oxygen reductioncatalyst.

In embodiments of the above catalyst and method for producing same, thepyrolysis is performed in a nitrogen-containing reactive gas or vapor.In specific embodiments, the nitrogen-containing reactive gas or vaporis NH₃.

In embodiments of the above catalyst and method for producing same, thepyrolysis is performed in an inert gas. In embodiments, the inert gas isargon.

In specific embodiments of the above catalyst and method for producingsame, a second pyrolysis in a nitrogen-containing reactive gas or vaporis performed following the pyrolysis performed in the inert gas. Inembodiments, the nitrogen-containing reactive gas or vapor for thesecond pyrolysis is NH₃.

In embodiments of the above catalyst and method for producing same, thepyrolysis (either the first and/or the second one if applicable) isperformed at a temperature greater than about 600° C.

For certainty, in embodiments of the above method for producing acatalyst, the catalyst precursor and/or the catalyst are as describedabove.

DETAILED DESCRIPTION OF THE INVENTION

It is an object of the present invention to provide non-precious(non-noble) metal catalysts (NPMCs) for the oxygen reduction reaction(ORR) in polymer electrolyte membrane fuel cells (PEMFCs). Such catalystmay be referred to as a catalyst of the type metal/N/C.

The present inventors were able to increase the number of catalyticsites on the carbon of such NPMCs using a microporous carbon (which hadalready lost most of its disordered carbon during its manufacturing), asa starting material. More specifically, the micropores of thismicroporous carbon were filled with a pore-filler (and non-noble metalprecursors). When reacting during subsequent pyrolysis, the pore-fillerand non-noble metal precursor produce catalytic sites exactly where theyare needed, i.e. in the micropores of the carbon substrate.

The obtained catalysts thus contain a high density of active sites.There may be different kinds of active sites in a same catalyst but allactive sites are believed to be as follows. First, because of the use ofpore-filler, each active site contains a carbon poly-aromatic structurewhose carbon atoms originate from the pore-filler (see FIG. 1). Theactive sites contain at least one non-noble metal atom and when there ismore than one non-noble metal atom, these non-noble metal atoms can beof same or different nature. The active sites also contain at leastabout four nitrogen atoms. Without being bound by theory, it is believedthat the nitrogen atoms are bound to the carbon atoms originating fromthe pore-filler and/or to the metal atom(s), resulting in pyridinic-typeor pyrrolic-type N atoms. It is also believed that the center of eachactive site is somewhat similar to the center of porphyrin orphthalocyanine molecules, for which all nitrogen atoms are of thepyrrolic-type. Finally, it is believed that the active sites have anelectronic contact with the walls of the micropores (see FIG. 1).

The tests presented below have revealed that catalysts obtained usingthis procedure have an activity in a fuel cell of 10 to 100 times higherthan prior art catalysts obtained by simple adsorption or impregnationon carbon. It is believed that the components that have beensuccessfully used in such prior art catalysts that were obtained bysimple adsorption or impregnation (such as the microporous supports, thenon-noble metal precursor, the pyrolysis gas, and the like) will also beuseful in the catalysts of the invention.

Turning now to the present invention in more detail, there is provided acatalyst precursor comprising: (A) a microporous support; (B) anon-noble metal precursor; and (C) a pore-filler, wherein the microporesof the microporous support are filled with the pore-filler and thenon-noble metal precursor so that the micropore surface area of thecatalyst precursor is substantially smaller than the micropore surfacearea of the support when the pore-filler and the non-noble metalprecursor are absent.

As used herein a “catalyst” means a substance that initiates orfacilitates a chemical reaction; a substance that boosts the kinetics ofa given reaction. A “catalyst precursor” is a substance from which acatalyst can be produced by pyrolysis. Herein, “pyrolysis” means thetransformation of a substance into one or more other substances by heatin the presence or absence of a gas (vacuum). As explained above, in thepresent invention, the pore-filler in the catalyst precursor reactduring pyrolysis to produce the desired catalytic sites in the catalyst.

As noted above, the catalyst precursors of the invention comprise amicroporous support. As used herein, a microporous support is a supportcomprising micropores. For example, a microporous support may have amicropore surface area of more than about 100 m²/g. Herein, “micropores”refer to pores having a size ≦2 nm. Most microporous supports usuallyalso comprise mesopores (between 2 and 50 nm in size) and macropores(having a size >50 nm). As such, microporous supports have a “total”surface area, which is provided by the micropores, the mesopores and themacropores. As used herein the “micropore surface area” of a substanceis the surface area of this substance provided by its micropores. The“total” surface area, micropore surface area, mesopore surface area andmacropores surface area can be determined by methods well known in theart. For example, by measuring the N₂-adsorption isotherm and analyzingit with the Brunauer Emett Teller (BET) equation and by applyingnon-local density functional theory using a slit-pore model(Quantachrome software) to determine pore size distribution.

In embodiments, the microporous support is a highly microporous support.For example, a “highly microporous support” may be a microporous supporthaving a micropore surface area of more than about 500 m²/g.

As will be appreciated by the person of skill in the art, the exactnature of the microporous support is of little importance to theinvention as long as it comprises micropores. When the catalyst is foruse in fuel cells, the microporous support will be conductive or able tobe made conductive by methods known to persons of skill in the art. Anon-conductive microporous support may be used, if it can be removedafter fabrication of the catalytic sites in its micropores, e.g. by acidleaching of the non-conductive support, while leaving intact a skeletonof carbon-based material with high site density. A non-limiting exampleof such a process is the use of microporous silicon dioxide and its acidleaching by hydrofluoric acid (HF). Hydrofluoric acid is known to notattack such non-noble metal active sites (REFERENCE 49). Anothernon-limiting example of conductive microporous support is a metalorganic framework (MOF). The latter will decompose upon pyrolysis andthus leave the desired skeleton of carbon-based material.

In embodiments, the microporous or highly microporous supports arecarbon-based. Such carbon-based supports can be carbon blacks, activatedcarbons, carbon nanotubes or nanostructures, carbons derived frommetallic carbides or oxides or from the pyrolysis of polymers, or fromany other method used to obtain highly microporous carbons. In otherembodiments, metal organic frameworks may also be used as microporoussupports. All the microporous supports may be used as mixtures ifdesired.

In embodiments, the microporous support may be a nitrogen precursor. Asused herein, a “nitrogen precursor” is a molecule or substance thatprovides one or more nitrogen atoms to the catalyst after pyrolysis.

As noted above, the catalyst precursors of the invention comprise anon-noble metal precursor. It is to be understood that a mixture ofnon-noble metal precursors can be used. Any non-noble metal precursorknown to the skilled person to be useful in catalysts of the prior art(i.e. those produced by adsorption or impregnation) may be used.

As used herein, a “non-noble metal” is a metal other than a noble metal.Noble metals are usually considered by the persons of skill in the artto be ruthenium, rhodium, palladium, osmium, iridium, platinum, andgold.

Examples of the non-noble metal include metals having atomic numbersbetween 22 and 32, between 40 and 50 or between 72 and 82, with theexclusion of atomic numbers 44-47 and 75-79. In embodiments, thenon-noble metal is iron, cobalt, copper, chromium, manganese or nickel.In specific embodiments, the non-noble metal is iron or cobalt. In morespecific embodiments, the non-noble metal is iron.

In embodiments, the catalyst precursor comprises between about 0.05 andabout 5.0 wt % of the non-noble metal based on the total weight of thecatalyst. In embodiments, the catalyst precursor has a non-noble metalcontent, as provided by the non-noble metal precursor, of about 0.2,0.5, 1.0, 2.5, 3.0, 3.5, 4.0, or 4.5 wt % or more based on the totalweight of the catalyst. In embodiments, the catalyst precursor has anon-noble metal content, as provided by the non-noble metal precursor,of about 5.0, 4.5, 4.0, 3.5, 3.0, 2.5, 2.0, 1.5, 1.0, or 0.5 wt % orless based on the total weight of the catalyst.

In embodiments, the catalyst precursor has an iron loading of about 0.2wt % or more based on the total weight of the catalyst precursor. Inmore specific embodiments, the catalyst precursor has an iron loading ofabout 1 wt % based on the total weight of the catalyst precursor.

As used herein, a “non-noble metal precursor” is a molecule thatprovides a non-noble metal atom to the catalyst during pyrolysis. It isto be understood that a non-noble metal precursor may contain only onenon-noble metal or a mixture of several non-noble metals. As notedabove, the active sites of the catalyst of the invention comprise atleast one non-noble metal atom.

The non-noble metal precursor may be organometallic or inorganic.

In embodiments, the non-noble metal precursor may be a salt of thenon-noble metal or an organometallic complex of the non-noble metal.Generally, any metal precursor of size <2 nm, or more generally of asize that allows it to be inserted or forced in the micropores, would besuitable for use in the present invention. Non-limiting examples ofnon-noble metal precursors include the following broad classes with morespecific examples in each class given between parentheses:

-   -   Metal acetates (Fe^(II) acetate, cobalt acetate, copper acetate,        chromium acetate, manganese acetate, nickel acetate);    -   Metal acetylacetonate (Fe^(II) acetylacetonate);    -   Metal sulfates (Fe^(II) sulfate);    -   Metal chlorides (Fe^(III) chloride);    -   Metal nitrates (Fe^(III) nitrate);    -   Metal oxalates (Fe^(II) oxalate);    -   Metal citrates (Fe^(III) citrate);    -   Fe(II) ethylene diammonium sulfate;    -   Metal porphyrins (Fe tetramethoxyphenylporphyrin, Fe        4-hydroxy-phenyl porphyrin, mesotetra-phenyl Fe porphyrin,        octaethyl Fe porphyrin, Fe pentafluorophenyl porphyrin);    -   Metallocene (Ferrocene, cobaltocene);    -   Metal-phthalocyanine (cobalt phthalocyanine, iron        phthalocyanine);    -   Tetra-aza-annulene (cobalt tetra-aza-annulene);    -   Metal oxides;    -   Metal nitrides;    -   Metal carbides;    -   Metal sputtered over the microporous support; and    -   Mixtures of the above.

Other non-limiting examples of non-noble metal precursors include:

-   -   cobalt porphyrin: Co tetramethoxyphenylporphyrin (TMPP);    -   iron acetate, Fe tetramethoxyphenylporphyrin (TMPP) on pyrolysed        perylene-tetracarboxylic-dianhydride (PTCDA);    -   Fe phthalocyanines;    -   Fe and Co tetraphenylporphyrin;    -   Co phthalocyanines;    -   Mo tetraphenylporphyrin;    -   metal/poly-o-phenylenediamine on carbon black;    -   metal porphyrin;    -   molybdenum nitride;    -   cobalt ethylene diamine;    -   hexacyanometallates;    -   pyrrol, polyacrylonitrile and cobalt;    -   cobalt tetraazaannulene; and    -   cobalt organic complexes.

In specific embodiments, the non-noble metal precursor is Fe^(II)acetate (FeAc).

In embodiments, the non-noble metal precursors may also be a nitrogenprecursor.

As noted above, the catalyst precursors of the invention comprise apore-filler. It is to be understood that a mixture of pore-fillers canbe used. It is also to be understood that the non-noble metal precursormay be the pore-filler, i.e. the non-noble metal precursor and thepore-filler may be the same molecule.

As used herein, a “pore-filler” is a molecule that (1) is small enoughto enter (or be forced in) and fill the micropores of the microporoussupport and (2) is carbon-based (i.e. organic) so that it reacts duringpyrolysis to produce in the micropores a carbon poly-aromatic structurewhose carbon atoms originate from the pore-filler as illustrated inFIG. 1. As will be appreciated by the person of skill in the art, theexact nature of the pore-filler has therefore little importance to thepresent invention as long as the pore-filler fulfills the above-notedrequirements and roles.

In embodiments, the pore-filler may comprise a polycyclic structure,i.e. a structure made of rings (loops formed by a series of connectedcarbon atoms), preferably aryl rings such as C₆ rings, for examplebenzene. These rings may more easily construct active sites and extendthe graphite platelets that are found on the edge of the graphiticcrystallites within the microporous support to provide the desiredcarbon poly-aromatic structure in the micropores of the microporoussupport.

Different types of pore-fillers may be used. A first type comprisesmolecules that contain carbon, but that do not contain nitrogen atoms.Non-limiting examples of classes of such pore-fillers include polycyclicaromatic hydrocarbons or their derivatives. Non-limiting examples ofpore-filler in these classes include perylene or perylenetetracarboxylic dianhydride.

A second type of pore-filler comprises molecules that contain bothcarbon and nitrogen atoms in their structure. Non-limiting examples ofclasses of such pore-fillers include phenanthrolines, melamine andcyanuric acid.

A last type of pore-filler comprises molecules that contain carbon,nitrogen atoms and at least one metal atom in their molecular structure.Non-limiting examples of classes of such pore-fillers includemetal-phenanthroline complexes, metal-phthalocyanines, andmetalporphyrins.

It is to be understood that the pore-filler may be any combination ofpore-fillers from the first, second and/or third above-described typesof pore-fillers.

In embodiments, the pore-filler may be a nitrogen precursor.

Non-limiting examples of pore-fillers that also are nitrogen precursorsinclude the following broad classes with specific examples given betweenparenthesis:

-   -   Phenanthroline (1,10-phenanthroline, Bathophenanthroline        disulfonic acid disodium salt hydrate, 4,7-Diphenyl and        5,6-dimethyl phenanthroline, 4-aminophenanthroline);    -   Phthalocyanine;    -   Porphyrine;    -   Pyrazine (Tetra 2 pyridinyl pyrazine, dihydropyridylpyridazine);    -   Phthalonitrile (4-Amino-phthalonitrile);    -   Pyridine (2,2′:6′,2″-Terpyridine,        4′-(4-Methylphenyl)-2,2′:6′,2″-terpyridine,        6,6″-Dibromo-2,2′:6′,2″-terpyridine,        6″-Dibromo-2,2′:6′,2″-terpyridine, aminopyridines)    -   Melamine;    -   Tetra-aza-annulene;    -   Hexaazatriphenylene;    -   Tetracarbonitrile;    -   Benzene-1,2,4,5-tetracarbonitrile    -   6-Pyridin-2-yl-[1,3,5]triazine-2,4-diamine;    -   All amino-acids;    -   Polypyrrole; and    -   Polyaniline.

Non-limiting examples of pore-fillers that do not contain nitrogen atomsand are thus not nitrogen precursors include the following broad classeswith specific examples given between parenthesis

-   -   Perylene [perylene-tetracarboxylic-dianhydride (PTCDA)];    -   Cyclohexane;    -   Benzene;    -   Toluene;    -   Pentacene;    -   Coronene;    -   Graphite transformed into disordered carbon of size <2 nm by        ball-milling;    -   Polycyclic aromatics (including perylene, pentacene, coronene,        etc.); and    -   Coal tar or petroleum pitch (these are raw materials for a        commercial process for carbon fiber production and are high in        polycyclic aromatics).

In embodiments, the pore-filler is perylene-tetracarboxylic-dianhydride,1,10-phenanthroline, perylene tetracarboxylic-diimide, orpolyacrylonitrile.

As described above, the pore-filler enters and fills the micropores ofthe microporous support. In the catalyst precursor of the invention, themicropores of the microporous support are filled with the pore-fillerand the non-noble metal precursor. As used herein, “the micropores arefilled” does not mean that all the micropores are completely full, itrather means that a substantial portion of the micropores are at leastpartially filled. The micropores may be considered as being filled iffor example 75% or more of them are completely full and 25% or less ofthem are empty or if 75% or more of them is mostly (at least half) full.

One observable effect of the micropores being filled is that themicropore surface area of the catalyst precursor becomes substantiallysmaller than the micropore surface area of the support when thepore-filler and the non-noble metal precursor are absent. Indeed, as canbe seen from FIG. 1 and FIG. 4, filled (or even partially filled)micropores have a smaller surface area than empty micropores. Therefore,the micropore surface area of the catalyst precursor where themicropores are filled will be substantially smaller than the microporesurface area of the microporous support with empty micropores.

In embodiments, the micropore surface area of the catalyst precursor isat most about 75% of the micropore surface area of the support when thepore-filler and the non-noble metal precursor are absent. In morespecific embodiments, the micropore surface area of the catalystprecursor is at most about 60%, 50%, 40%, 30%, 20%, or 10% of themicropore surface area of the support when the pore-filler and thenon-noble metal precursor are absent.

In embodiments, the pore-filler/microporous support mass ratio is 50:50(as calculated using the weight of the pore-filler and the weight of themicroporous support in the catalyst precursor).

The present invention also relates to a catalyst comprising the abovecatalyst precursor, wherein the catalyst precursor has been pyrolysed sothat the micropore surface area of the catalyst is substantially largerthan the micropore surface area of catalyst precursor, with the provisothat the pyrolysis is performed in the presence of a gas that is anitrogen precursor when the microporous support, the non-noble metalprecursor and the pore-filler are not nitrogen precursors.

As explained above, it is believed that the active catalytic sites ofthe present catalyst comprise a carbon poly-aromatic structure whosecarbon atoms originate from the pore-filler, at least one non-noblemetal atom and at least four nitrogen atoms. There must therefore benitrogen atoms provided to the catalyst. These nitrogen atoms can beprovided by the microporous support, the non-noble metal precursor, thepore-filler and/or the pyrolysis gas. In this case, these components arealso nitrogen precursor as explained above.

When the microporous support, the non-noble metal precursor and thepore-filler are not nitrogen precursors, the necessary nitrogen atomsare provided by a gas used during pyrolysis. Therefore in that case, thegas itself is a nitrogen precursor.

As explained above, in the present invention, the pore-filler of thecatalyst precursor is believed to react during pyrolysis to produce thedesired catalytic sites in the catalyst. More specifically, pyrolysis ofthe catalyst precursor causes the pore-filler to react and produce acarbon poly-aromatic structure whose carbon atoms originate from thepore-filler in the micropores. This results in the construction ofactive catalytic sites and the extension of the graphite platelets thatare found on the edge of the graphitic microporous support. Thepyrolysis also causes the non-noble metal precursor and the nitrogenprecursor (be it the microporous support, the non-noble metal precursor,the pore-filler or the gas used for pyrolysis) to react and give uptheir non-noble metal and nitrogen atoms to the catalytic site. Theactive catalytic sites are thus formed from the carbon from thepore-filler, the nitrogen from the nitrogen precursor and the non-noblemetal from the non-noble metal precursor. This whole process isillustrated in FIG. 1. More detail on the pyrolysis procedure will begiven below.

As can be seen from FIG. 1, as the pore-filler, the nitrogen precursorand the non-noble metal react (i.e. decompose and partially go away)during pyrolysis, the micropores of the microporous support are more orless restored. Therefore, the micropore surface area of the catalystbecomes substantially larger during pyrolysis. In other words, themicropore surface area of the catalyst is substantially larger than themicropore surface area of catalyst precursor. In the extreme, themicropore surface area of the catalyst may be almost as high as themicropore surface area of the microporous support when pore-filler andthe non-noble metal precursor were absent.

In embodiments, the micropore surface area of the catalyst is at leastabout 50% of the micropore surface area of the support when thepore-filler and the non-noble metal precursor are absent. Inembodiments, the micropore surface area of the catalyst is at leastabout 60%, 70%, 75% or 80% of the micropore surface area of the supportwhen the pore-filler and the non-noble metal precursor are absent.

As such, it should be understood that the pyrolysis cause a loss ofmass. The catalyst obtained after pyrolysis is lighter than the catalystprecursor. In embodiments, the mass loss during pyrolysis is about equalto a pore-filler loading in the catalyst precursor (in weight % based onthe total weight on the catalyst precursor).

The non-noble metal content of the catalyst after pyrolysis may bemeasured by methods known in the art, for example neutron activationanalysis.

The catalyst may comprise between about 0.5 to about 10.0 wt % of thenitrogen based on the total weight of the catalyst. In embodiments, thecatalyst has a nitrogen content, as provided by the nitrogen precursor,of about 0.5, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, or 9.0 wt % ormore based on the total weight of the catalyst. In embodiments, thecatalyst has a nitrogen content, as provided by the nitrogen precursor,of about 9.0, 8.0, 7.0, 6.0, 5.0, 4.0, 3.0, 2.0, 1.0 wt % or less basedon the total weight of the catalyst. This nitrogen content may bemeasured by methods known in the art, for example, x-ray photoelectronspectroscopy.

If the microporous support is carbon-based, the carbon content isusually about 80 wt % or more based on the total weight of the catalyst.The catalyst may comprise between about 80 and about 99.9 wt % ofcarbon. It is to be noted that carbon usually comprises some oxygen(usually between 0.5 and 5% wt). If the microporous support does notcontain carbon, the carbon content of the catalyst may be low since thecarbon content will be provided only by from the pore-filler (andoptionally the non-noble metal precursor) used to fill the microporoussupport.

In embodiments, the catalyst is an oxygen reduction catalyst, a catalystfor the disproportionation of hydrogen peroxide or a catalyst for thereduction of CO₂. Indeed, it is believed that the present catalysts willbe useful for the disproportionation of hydrogen peroxide and thereduction of CO₂ because it is known for non noble metal catalystsobtained from heat treatment or without heat treatment (metal-N₄molecules as phthalocyanines) that the activity for the O₂electro-reduction reaction and for the chemical disproportionation ofH₂O₂ follow the same trend, i.e. if a catalyst shows high activity forone reaction, it will show high activity for the other reaction as well(REFERENCE 50). This fact has also been verified by the presentinventors and improved H₂O₂ disproportionation reaction has beenmeasured on a catalyst of the invention. Further, it is also known thatelectroreduction of CO₂ is catalyzed by metal macrocycles in which ametal ion is coordinated to 4 nitrogen atoms located in a polyaromaticframe, a structure similar to that proposed for the present catalyticsites used for the reduction of oxygen (REFERENCE 51).

In more specific embodiments, the catalyst is an oxygen reductioncatalyst. Such a catalyst will be useful at the cathode of various lowtemperature fuel cells, including principally polymer electrolytemembrane (PEM) such as H₂/O₂ polymer electrolyte membrane fuel cells,direct alcohol fuel cells, direct formic acid fuel cells and evenalkaline fuel cells. Such a catalyst may also be useful at the cathodeof various primary and secondary metal-air batteries, including zinc-airbatteries.

The present invention also relate to methods of producing theabove-described catalyst precursors and catalysts.

Therefore, the present invention relates to a method for producing acatalyst precursor, the method comprising (A) providing a microporoussupport; a non-noble metal precursor; and a pore-filler; and (B) fillingthe micropores of the microporous support with the pore-filler and thenon-noble metal precursor so that the micropore surface area of thecatalyst precursor is substantially smaller than the micropore surfacearea of the support when the pore-filler and the non-noble metalprecursor are absent.

It should be noted that prior art methods of adding nitrogen and/ornon-noble metal precursors to the support did not result in the fillingof the micropores as in the present invention. Indeed, the prior artmethods for adding these nitrogen and/or non-noble metal precursors tocatalyst precursors typically involved impregnation or adsorption. Thereare inherent solubility and adsorbability limitations to these methodsthat prevent the filling of the micropores so that that the microporesurface area of the catalyst precursor is substantially smaller than themicropore surface area of the support when the pore-filler and thenon-noble metal precursor are absent. Furthermore, in the prior art, thenitrogen and/or non-noble metal precursors were sometimes added tonon-microporous supports to produce catalyst precursors.

In the present invention, the addition of the pore-filler and thenon-noble metal precursors to the microporous support is carried out sothat the micropores of the support are filled and the micropore surfacearea of the catalyst precursor is substantially smaller than themicropore surface area of the support when the pore-filler and thenon-noble metal precursor are absent. The limitation associated with theprior art impregnation/adsorption methods are thus overcome as themicropores filled using different methods. Non-limiting examples of suchmethods include any form of ballmilling or reactive ballmilling,including but not limited to planetary ballmilling, and resonantacoustic mixing.

Planetary ballmilling is a low-energy material processing techniqueinvolving a container with grinding media that rotates in a planet-likemotion. It uses both friction and impact effects to force thepore-filler and the non-noble metal precursor into the micropores of themicroporous support, while leaving its microstructure relativelyunaffected. The ballmilling may be performed on dry powders of non-noblemetal precursor, the pore-filler and the microporous support.Alternatively, ballmilling may be performed in wet conditions with thenon-noble metal precursor and the pore-filler in solution and themicroporous support in suspension in this solution.

Resonant mixing is a method that uses low-frequency high-intensity soundenergy for mixing. It may be carried out with or without grinding media.

Therefore, in embodiments of the present invention, the micropores ofthe microporous support are filled with the pore-filler and thenon-noble metal precursor by ballmilling or by acoustic mixing with orwithout grinding media. In more specific embodiments, the ballmilling isplanetary ballmilling.

It is to be understood that the non-noble metal precursor and thepore-filler may be introduced in the micropores of the microporoussupport either together or separately.

The present invention also relates to a method of producing a catalyst,the method comprising (A) providing a catalyst precursor comprising amicroporous support; a non-noble metal precursor; and a pore-filler,wherein the micropores of the microporous support are filled with thepore-filler and the non-noble metal precursor so that the microporesurface area of the catalyst precursor is substantially smaller than themicropore surface area of the support when the pore-filler and thenon-noble metal precursor are absent; and (B) pyrolyzing said catalystprecursor so that the micropore surface area of the catalyst issubstantially larger than the micropore surface area of catalystprecursor, with the proviso that the pyrolysis is performed in thepresence of a gas that is a nitrogen precursor when the microporoussupport, the non-noble metal precursor and the pore-filler are notnitrogen precursors.

The atmosphere in which the pyrolysis is performed may be:

-   -   a nitrogen-containing reactive gas or vapor, non-limiting        examples of which being NH₃, HCN, and CH₃CN;    -   an inert gas, non-limiting examples of which being N₂, Ar, and        any other inert gas or vapor; or    -   a mixture of a nitrogen-containing reactive gas or vapor and an        inert gas.

As used herein a nitrogen-containing reactive gas or vapor is anitrogen-containing gas or vapor that will react during pyrolysis toprovide a nitrogen atom to the catalyst. A non-limiting example of anitrogen-containing gas or vapor that does not so react is N₂.

As used herein, an inert gas is a gas that will not react with thecatalyst precursor/catalyst at the pyrolysis temperature, an example ofwhich is argon.

Therefore, in embodiments the pyrolysis is performed in anitrogen-containing reactive gas or vapor. In specific embodiments, thenitrogen-containing reactive gas or vapor is NH₃.

In other embodiments, the pyrolysis is performed in an inert gas. Inembodiments, the inert gas is argon.

In embodiments where the pyrolysis is performed in an inert gas, asecond pyrolysis in a nitrogen-containing reactive gas or vapor isperformed following the pyrolysis performed in the inert gas.

As explained above however, there must be nitrogen atoms provided to thecatalyst. These nitrogen atoms can be provided by the microporoussupport, the non-noble metal precursor, the pore-filler and/or the gasused for pyrolysis. When the microporous support, the non-noble metalprecursor and the pore-filler are not nitrogen precursors, the necessarynitrogen atoms must be provided by the gas used during pyrolysis (eitherthe first one, or the second if present). Therefore in that case, thepyrolysis gas itself is a nitrogen precursor. In specific embodiments,the gas that is a nitrogen precursor is a nitrogen-containing reactivegas or vapor, non-limiting examples thereof being NH₃, HCN, and CH₃CN.

The time and temperature required for the pyrolysis will be easilydetermined by the person of skill in the art. More specifically, thepyrolysis may be performed at temperatures ranging from about 300 toabout 1200° C. In specific embodiments, the pyrolysis is performed at atemperature greater than about 600° C.

Optionally, a final pyrolysis under H₂ may be carried out to eliminateexcess heteroatoms like (N, O, S, etc. . . . ) in the catalyst ifdesired.

Optionally, acid etching may be used to remove excess non-noble metalbefore or after any pyrolysis.

Post-pyrolysis ballmilling can optionally be carried out to controlparticle size formed during the pyrolysis.

For use in a fuel cell, the catalyst is processed in order to form partof the cathode of the fuel cell. This is typically accomplished bythoroughly mixing the catalyst and an ionomer like Nafion. The Nafion tocatalyst mass ratio has to be adjusted and depends on the catalyst, butcan be easily determined by the person of skill in the art. The optimalratio may range between about 1 and about 4. Given the fact that thepresent catalysts are much less expensive than noble metal orplatinum-based catalysts, the current density of the fuel cell may beincreased by increasing the loading of the former with little effect oncost. Therefore, the loading of present catalysts may be increased untilmass transport losses become unacceptable.

If the electron conductive properties of the obtained catalysts are notsufficient for optimal performance in fuel cell, a given ratio of aconductive powder [carbon black or any electron conductive powder thatdoes not corrode in acid medium (for all PEM fuel cells) or alkalinemedium (for alkaline fuel cell)] may be added.

Herein, “about” means plus or minus 5% of any numerical value itqualifies.

Other objects, advantages and features of the present invention willbecome more apparent upon reading of the following non-restrictivedescription of specific embodiments thereof, given by way of exampleonly with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the appended drawings:

FIG. 1 is a schematic representation of catalytic site formation in themicropores of the carbon support: (a) Simplified 3-D view of a slit porebetween two adjacent graphitic crystallites in the carbon support; (b)Plan view of an empty slit pore between two crystallites; (c) Plan viewof a slit pore filled with pore-filler and metal precursor afterplanetary ballmilling; and (d) Plan view of the presumed catalytic site(incomplete) and graphene layer growth (shaded aromatic cycles) betweentwo crystallites after pyrolysis.

FIG. 2 shows the volumetric current density of best non-precious metalcatalyst described below. Polarization curves (converted to P_(O2) 1bar, 100% RH, 80° C.) from H₂—O₂ fuel cell testing for cathodes madewith the best non-precious metal catalyst (NPMC) below (hollow circles)and for reference purposes, the presumed previous best NPMC (REFERENCE7) (hollow diamonds). A catalyst loading of ca. 1 mg·cm⁻² was used forboth NPMC polarization curves. The actual Fe content in the catalystfrom below is 1.7 wt %, resulting in a Fe loading of 17 μg·cm⁻². Thevolumetric (kinetic: free of diffusion or charge transport effects thatare not related to the catalytic activity of the catalyst) currentdensity at 0.8 V iR-free cell voltage is the intersection of theextended Tafel slope of the polarization curves (dashed lines) with the0.8 V axis. Also included are the 2010 (filled star) and 2015 (filledhexagon) U.S. DOE performance targets for ORR on NPMCs, all at thereference conditions of PO2 1 bar, 100% RH and 80° C.

FIG. 3 is a comparison of best non-precious metal catalyst describedbelow with a Pt-based catalyst. Polarization curves from H₂—O₂ fuel celltesting (P_(O2) and P_(H2) 1.5 bar, 100% RH, 80° C.) for cathodes madewith the best non-precious metal catalyst below (two different catalystloadings) and a ready-to-use Gore PRIMEA™ 5510 MEA from W. L. Gore &Associates with ca. 0.4 mg Pt cm⁻² at cathode and anode (top line asindicated). Flow rates for H₂ and O₂ were well above stoichiometric. Theactual Fe content in our catalyst is 1.7 wt %, resulting in a Fe loadingof 17 μg·cm⁻² for a catalyst loading 1 mg cm⁻².

FIG. 4 shows the micropore surface area (hollow squares) and kineticcurrent at 0.8 V_(iR-free) cell voltage obtained from fuel cell testing(hollow circles) vs mass loss (during pyrolysis) for catalysts producedwith a 50% nominal concentration of PCTDA as the pore-filler and anominal Fe content of 0.2 wt %. The kinetic current, in A·g⁻¹ at0.8V_(iR-free) under 1.5 bar O₂ corresponding to the optimal mass lossupon pyrolysis, is reported in Table 2. Black Pearls 2000™ was used asthe carbon support. Pyrolysis was conducted in NH₃ at 1050° C. Catalystloading for all tests was ca. 1 mg·cm⁻². The Nafion-to-Catalyst ratiowas 2. Micropore surface areas of pristine and pore-filled carbonsupport are represented by the filled triangle and filled star,respectively.

FIG. 5 shows polarization curves from fuel cell testing (P_(O2) 1.5 bar,100% RH, 80° C.) for catalysts produced with a nominal Fe content of 0.2wt % and various nominal concentrations of PTCDA as the pore-filler: 0%(fourth line from the top when looking at the left side of the graphic,25% (third line), 50% (second line) and 75% (first line). Black Pearls2000™ was used as the carbon support. Pyrolysis was conducted in NH₃ at1050° C. Catalyst loading for all tests was ca. 1 mg·cm⁻². TheNafion-to-Catalyst ratio was 2.

FIG. 6 shows the kinetic activity at 0.8V iR-free, nitrogen content, andmicropore surface area for catalysts made with 50% BP, 50% PTCDA and 1wt % of iron, pyrolysed using method I (1050° C. in NH₃).

FIG. 7 shows the nitrogen content, micropore surface area and ORRkinetic activity in fuel cell for catalysts made using method I (NH₃pyrolysis at 1050° C.) having (A) various mass ratios of PTCDA in thecatalyst precursor and a fixed nominal iron loading of 0.2 wt %; and (B)fixed mass ratio of 50 wt % PTCDA in the catalyst precursor with variousnominal iron loadings. Upper graphs: nitrogen content (squares, read onleft hand-side Y-scale) and micropore surface area (stars, read on righthand-side Y-scale). Lower graphs: Kinetic current per mass of catalystmeasured at 0.9V iR-free voltage (upright triangles) and 0.8V iR-freevoltage (inverted triangles).

FIG. 8 shows the nitrogen content, micropore surface area and ORRkinetic activity in fuel cell for catalysts made using method II (Arpyrolysis at 1050° C. for 60 min and then NH₃ pyrolysis at 950° C.)having (A) various mass ratios of phen in the catalyst precursor and afixed nominal iron loading of 1 wt %; and (B) fixed mass ratio of 50 wt% phen in the catalyst precursor with various nominal iron loadings.Upper graphs: nitrogen content (squares, read on left hand-side Y-scale)and micropore surface area (stars, read on right hand-side Y-scale).Lower graphs: Kinetic current per mass of catalyst measured at 0.9ViR-free voltage (upright triangles) and 0.8V iR-free voltage (invertedtriangles).

FIG. 9 shows the PEM fuel cell polarization curves for Fe-basedcatalysts made with various combinations of carbon black/pore-filler. Inall cases the pore-filler mass ratio and nominal iron content was 50%and 1 wt %, respectively. Inset: iR-free voltage vs. current per cm⁻²,semi-logarithmic plot. All catalyst precursors were subjected to onepyrolysis in NH₃ at 1050° C. except the catalyst made with BP+phen. Thelatter was subject to two pyrolyses, the 1^(st) in Ar and the 2^(nd) inammonia (see experimental). H₂/O₂, 1 bar back pressure, 100% relativehumidity, cell at 80° C., 1.14 cm² geometric area of electrode.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present invention is illustrated in further details by the followingnon-limiting examples.

EXAMPLE 1

The embodiments of the invention described below elevates the catalyticactivity of iron-based NPMCs by a factor >25 compared to the previouslybest reported activity (REFERENCE 7); high enough to equal Pt-basedcathodes with loadings ≦0.4 mgPt·cm⁻², at cell voltage ≧0.9 V. Theresults presented below show that sufficiently active and inexpensiveNPMCs for the ORR are possible. The present NPMCs will be useful for ORRin direct alcohol, formic acid and alkaline fuel cells.

To capitalize on the high micropore content of microporous carbon blacksand overcome the limitation due to their lack of disordered carbon,these micropores were filled with a mixture of pore-filler (PF) and ironprecursor. Doing so creates a catalyst precursor that complies with thefour factors required for producing active NPMCs, as described above.This innovative concept is illustrated in FIG. 1. To overcome thelimitation of solubility and/or adsorbability associated with theimpregnation method, planetary ballmilling was used to fill the pores ofthe microporous carbon support with various PFs and metal precursor.

In the present, the chosen microporous carbon black (micropore surfacearea 934 m² g⁻¹) and iron precursor used for all catalysts is BlackPearls 2000™ by Cabot (BP) and iron^(II) acetate (FeAc), respectively.Two pore-fillers were used. The first,perylene-tetracarboxylic-dianhydride (PTCDA) is nitrogen free. Thesecond, 1,10-phenanthroline (Phen) is N-bearing. For catalysts madeusing PTCDA, the N atoms necessary to form catalytic sites arise fromits reaction with NH₃ during pyrolysis. For catalysts made using Phen,the pyrolysis was performed in either Ar or NH₃ as Phen already containsnitrogen. It is also worthy to note that Phen forms a complex with Fe²⁺.

Catalyst Synthesis. Catalyst precursors refer to the powder mixturesprepared to be subsequently pyrolysed. All prepared catalyst precursorsconsist of a carbon support, a pore-filler and a non-noble metalprecursor. These powder mixtures were prepared using the planetaryballmilling (PBM) method. The carbon support and the non-noble metalprecursor used for all catalysts are BP and FeAc, respectively. The PFsused are PCTDA or Phen, which are represented as molecules (a) and (b)below respectively.

For catalyst precursors containing PTCDA, the three materials (BP, PTCDAand FeAc) were placed in a hardened steel vial (ca. 65 cm³) with 20chrome steel balls of 0.25 inch diameter. Typically, ca. 1 g of powderis ballmilled at once. The ball-to-powder ratio was ca. 20:1. The vialwas purged of air and filled with nitrogen using a glovebox. Oncetightly sealed, the vial was placed in a planetary ballmiller (FRITSCHPulverisette 7) to undergo 3 hours of ballmilling at 400 rpm. Theresulting dry powder formed the catalyst precursor.

For catalyst precursors containing Phen, the Phen and FeAc were firstmixed in a solution of ethanol to form a [Fe(Phen)₃]²⁺complex. This wasevidenced by the deep red color that emerged from an otherwise clearsolution. The carbon support, BP, was then added to the solution. Thissolution was stirred over a magnetic hotplate for ca. 2 hours and thenplaced in a drying oven overnight to be completely dried in air at ca.90° C. Once dried the powder was placed in a steel vial and underwentthe same ballmilling processing steps as described earlier for thecatalyst precursor containing PCTDA. The resulting dry powder formed thecatalyst precursor.

To become active ORR catalysts all catalyst precursors underwent one ortwo pyrolyses in either Ar or NH₃ depending on the PF used. Thepyrolysis procedures to obtain the catalytic activities reported inTables 2 and 3 are described below.

Methods. Surface area measurement of catalysts was performed with aQuantachrome Instruments Autosorb-1 and with N₂ as an adsorbate.Isotherm analysis was performed as in REFERENCE 19. Surface elementalanalysis of catalysts was performed by X-ray photoelectron spectroscopyusing a VG Escalab 200i instrument. The Al K_(α) line (1486.6 eV) waschosen as the X-ray source. The quantification of the elements wasperformed using Casa software. Bulk Fe content of the best catalyst wasmeasured with neutron activation analysis at École Polytechnique deMontréal. MEAs were prepared using a Nafion 117 membrane andhot-pressing of the anode, cathode and membrane was done at ca. 140° C.for 40 seconds.

Electrochemical characterization. Two catalyst ink formulations wereused. One formulation resulted in a Nafion-to-catalyst ratio (NCR) of 2and another 1.5. For catalyst inks with a NCR of 2, 10 mg of catalystwas mixed in a glass vial with 435 μL, of Nafion 5 wt % (Aldrich), 54μL, of ethanol and 136 μL of nanopure water. For catalyst inks with aNCR of 1.5, 10 mg of catalyst was mixed in a glass vial with 326 μL ofNafion 5 wt % (Aldrich), 163 μL of ethanol and 136 μL of nanopure water.In both cases the inks were first sonicated for 30 minutes, agitated ina vortex mixer for 15 minutes, sonicated once more for 15 minutes andfinally agitated for 5 more minutes. These catalysts inks were then usedto prepare the cathode for fuel cell testing.

Preparation of Cathodes and Anodes. The cathodes used for fuel celltesting were prepared using the catalyst inks described above. To obtaina loading of ca. 1 mg·cm−2 of catalyst, 71 μL of ink was deposited on around uncatalysed 1.14 cm² substrate, or gas diffusion media (BASFELAT). For some fuel cell tests, however, higher loadings weredeposited. The anode used for all fuel cell tests performed with NPMCsat the cathode was the catalyst layer of a BASF ELAT substrate coatedwith 0.5 mg Pt·cm⁻², 20 wt % Pt/Vu. The active side (catalyst layer) ofthe substrate was brushed with a thin layer of Nafion 5 wt % solution(ca. 0.5 mg·cm⁻²). The anode and cathode were then placed in a vacuumoven at ca. 80° C. to dry for 1 hour. The mass of catalyst in thecathode was determined by subtracting the mass of the uncatalysedsubstrate from the dry mass of the catalyst-coated substrate anddividing the remainder by (NCR+1).

Fuel Cell Testing. Membrane electrode assemblies (MEAs) were tested in asingle-cell test fuel cell (Electrochem Inc.) and the experiments werecontrolled with a potentiostat PARSTAT™ 2273 (Princeton AppliedResearch).

Teflon gaskets were used at both the anode and cathode sides. The gasketthicknesses were chosen to obtain ca. 25% compression of the gasdiffusion+catalyst layers.

First, the fuel cell and H₂/O₂ humidifier temperatures were raised andmaintained to 80° C. and 105° C./95° C., respectively, under N₂ flows.Once set temperatures were reached, pure H₂ and O₂ were then fed for ca.15 minutes. Back pressures were set to ca. 1 bar for both anode andcathode sides. Thus, the absolute pressure for both H₂ and O₂ is 1.5bars (with ca. 0.5 bar partial pressure attributed to water vapor at ca.80° C.). Flow rates for H₂ and O₂ were well above stoichiometric.

First an electrochemical impedance spectroscopy (EIS) measurement wasmade at open circuit voltage (OCV), with frequencies ranging from 50 kHzdown to 10 Hz. Typical resistance values of ca. 0.2 Ω·cm² were obtained.A polarization curve was then recorded by scanning the cell voltage fromOCV down to 0 V at a scan rate of 0.5 mVs⁻¹.

Results and Discussion. First, the effect of the wt % of PTCDA in thecatalyst precursor was investigated. Four different wt % PTCDA (0, 25,50, 75) were used with a constant nominal Fe loading of 0.2 wt %.Optimal volumetric activities of 1.8, 8.5, 22, and 27 A·cm⁻³ wereobtained, respectively. The experimental conditions and correspondingfuel cell polarization curves are given in FIG. 5. Following theseexperiments, a pore filler loading of 50 wt % (PTCDA or Phen) was chosento investigate the effect of nominal Fe loading in the catalystprecursor. Catalyst precursors made with PTCDA were pyrolysed in NH₃ andthose made with Phen in Ar, both at 1050° C. While better volumetricactivities were obtained with the PTCDA series (Table 1), the effect ofa subsequent 5 minute pyrolysis in NH₃ for the Phen series wasinvestigated. This subsequent pyrolysis amplified the volumetricactivity of the Ar-pyrolysed Phen series up to 20 times. These amplifiedactivities surpass those of the PTCDA series. Two factors were furtheroptimized on the most active catalyst corresponding to 1 wt % nominal Fecontent (84 A·cm⁻³, Table 1): (i) the mass loss during pyrolysis in NH₃and (ii) the effect of the Nafion-to-catalyst ratio (NCR) in thecathode. The optimal mass loss and NCR were found to be ca. 30% and 1.5,respectively, leading to an increase in volumetric activity from 64 to99 A·cm⁻³, much closer to the 2010 U.S. DOE performance target for ORRon NPMCs. Additional details on mass activity, mass loss duringpyrolysis, micropore surface area and nitrogen content may be found inTable 2. Details of methods used for catalyst synthesis, MEA preparationand fuel cell testing may be found below.

TABLE 1 Catalytic activities for optimized catalysts. Activities at 0.8V vs RHE obtained from fuel cell testing at 80° C. and under 1.5 bar O₂.Black Pearls 2000 ™ was used as the carbon support. The mass ratio ofpore-filler to carbon support was 50/50. Catalyst loading for all testswas ca. 1 mg · cm⁻². The Nafion- to-Catalyst ratio (NCR) was 2 unlessotherwise noted. 1 st Pyrolysis 2nd Pyrolysis Nominal Fe Catalytic Gasand Gas and Pore- Content Activity (Temp.) (Temp.) Filler (wt %) (A ·cm⁻³)^(a) NH₃ (1050° C.) — PTCDA 0.2 22  NH₃ (1050° C.) — PTCDA 0.5 24 NH₃ (1050° C.) — PTCDA 1.0 30  Ar (1050° C.) — Phen 0.2   2.8 Ar (1050°C.) — Phen 1.0   5.5 Ar (1050° C.) — Phen 4.1   1.4 Ar (1050° C.) NH₃(950° C.) Phen 0.2 60^(b) Ar (1050° C.) NH₃ (950° C.) Phen 1.0 64^(b) Ar(1050° C.) NH₃ (950° C.) Phen 4.1 31^(b) Ar (1050° C.) NH₃ (950° C.)Phen 1.0 99^(c) (NCR 1.5) ^(a)Converted from the activity measured under1.5 bar O₂ absolute pressure to the reference pressure of 1 bar O₂absolute pressure (details available in the Supplementary Information).^(b)Unoptimized mass loss in 2^(nd) pyrolysis. ^(c)Optimized mass lossin 1^(st) and 2^(nd) pyrolysis

FIG. 2 presents the polarization curves in terms of volumetric currentdensity for our best NPMC and for the presumed best NPMC reported todate by Wood et al. (3M, 2008, REFERENCE 7). The U.S. DOE volumetricactivity target for ORR on NPMCs is specified for 0.8V iR-free cellvoltage. As shown in FIG. 2, the kinetic activity (free of masstransport losses) of the NPMCs at 0.8 V iR-free cell voltage cannot bedirectly read from the polarization curves, but must instead beestimated by extrapolating the kinetically controlled Tafel slopeobserved at higher cell voltage. Our best NPMC shows an activityenhancement of more that 25 times that of the previously highest NPMCactivity.

To give prominence to the progress made by the NPMCs herein for ORR inPEM fuel cells, FIG. 3 shows two polarization curves, in terms ofcurrent density (A·cm⁻²), of the best NPMC produced herein (Table 1);one using a catalyst loading of 1.0 and the other 5.3 mg·cm⁻². Thesepolarization curves are compared with that of a Pt-based cathodecatalyst (Gore PRIMEA™5510 MEA from W. L. Gore & Associates, ca. 0.4mg·cm⁻² Pt at cathode and anode) tested under the same conditions andtest fuel cell. It can be seen in FIG. 3 that at 0.9 V iR-free cellvoltage, within the kinetically controlled Tafel region for bothpolarization curves, increasing the loading of the NPMC by ca. 5increases the current density of the cell by about the same factor. Itcan also be seen at 0.9V that the current density of the NPMC (5.3mg·cm⁻²) is equal to that of the Pt-based catalyst. Although it may seemunfair to compare a NPMC loading of ca. 5 mg·cm⁻² with a Pt loading of0.4 mg·cm⁻², the limiting factor for the Pt loading is cost, while nosuch factor exists for the low-cost NPMCs herein. However, for currentdensities >0.1 A·cm⁻², the NPMC-based cathodes display lower performancethan the Pt cathode (FIG. 3). This is probably caused by poormass-transport properties that must be improved.

Steps followed to obtain the data appearing in Tables 2 and 3. Fourseries of catalyst were prepared using PCTDA as the pore-filler andBlack Pearls 2000™ as the carbon support corresponding to 4 differentPTCDA nominal concentrations; 0, 25, 50 and 75%. The nominal Fe contentfor all catalysts was 0.2 wt %. All pyrolyses were conducted either inAr or in NH₃ (as specified in the Table captions) at 1050° C.

Series 1.

-   -   a. For each concentration of PCTDA several catalysts were        produced by varying the pyrolysis time, each time resulting in a        different mass loss.    -   b. Fuel cell tests were conducted for each catalyst in 1a. A        Nafion-to-catalyst ratio of 2 was used for all tests. The        kinetic currents at 0.8 V_(iR-free) was determined by first        converting the current from A·cm⁻² to A·g⁻¹, then finding the        intersection of the extrapolated Tafel slope with the 0.8        V_(iR-free) axis. An example of such kinetic values vs. mass        loss is found in FIG. 4 (hollow circles) for the series of 10        catalysts prepared with 50% nominal PTCDA. The point        corresponding to the maximum catalytic activity at the optimal        mass loss is identified by an arrow in FIG. 4. This maximum        activity (97 A·g⁻¹) obtained in fuel cell testing at        0.8V_(iR-free) vs RHE and 80° C. is reported in Table 2 for 50%        nominal PTCDA. The polarization curve corresponding to this        optimal catalyst for 50% nominal PTCDA is depicted by the second        line from the top when looking at the left side of the graph in        FIG. 5.    -   c. Micropore surface area measurements were performed for        selected catalysts in 1. An example of such micropore surface        area values vs. mass loss (corresponding to the 50% nominal        PTCDA concentration series) is found in FIG. 4 (hollow squares).        Note how the maximum kinetic current in this series coincides        closely with the maximum micropore surface area, suggesting that        micropores play an important role in the formation of catalytic        sites.

A summary of the optimal polarization curves (in A·cm⁻²) correspondingto each of the four nominal PTCDA concentration series described aboveis found in FIG. 5. All values of catalytic activity (in A·g⁻¹, obtainedin fuel cell testing at 0.8 V_(iR-free) vs. RHE and 80° C.), optimalmass loss, micropore surface area and surface N content for the fourcatalysts shown in FIG. 5 are listed in Table 2. Finally, 50%pore-filler concentration was chosen to conduct the remainder of allexperiments herein.

TABLE 2 Catalytic activity (in A · g⁻¹, obtained in fuel cell testing at0.8 V_(iR-free) vs RHE and 80° C., under 1.5 bar O₂), optimal mass lossduring pyrolysis, micropore surface area and N content for catalystsproduced at optimal mass loss with different nominal concentrations ofPCTDA as the pore-filler and a nominal Fe content of 0.2 wt %. BlackPearls 2000 ™ was used as the carbon support. Pyrolysis was conducted inNH₃ at 1050° C. Catalyst loading for all tests was ca. 1 mg · cm⁻². TheNafion-to-Catalyst ratio was 2. The volumetric activities underreference conditions of 1 bar O₂ are obtained by multiplying thecatalytic activities measured at 1.5 bar O₂ (A · g⁻¹) by a factor of0.231 as explained in the following section. Nominal Catalytic OptimalMicropore N PCTDA Activity Mass Loss Surface Area Content (wt %) (A ·g⁻¹) (%) (m²g⁻¹) (at %) 0 8 30 741 — 25 37 30 864 1.1 50 97 45 716 1.975 116 66 738 3.2Series 2.

Three series of catalysts were produced using PCTDA as the pore-fillerand Black Pearls 2000™ as the carbon support with a 50 PTCDA nominalconcentration, corresponding to three different nominal Feconcentrations; 0.2 (already covered in 1.), 0.5 and 1 wt %. Allpyrolyses were conducted in NH₃ at 1050° C.

-   -   a. For each nominal Fe concentration several catalysts were        produced by varying the pyrolysis time, each time resulting in a        different mass loss.    -   b. Fuel cell tests were conducted for each catalyst in 2a. The        kinetic currents at 0.8 V_(iR-free) were determined as in 1b.        The kinetic currents, in A·g⁻¹ at 0.8V_(iR-free) vs RHE and        80° C. corresponding to the optimal mass loss upon pyrolysis,        are reported in Table 3.    -   c. Micropore surface area measurements were performed for the        optimal catalyst in each series.

All values of catalytic activity (in A·g⁻¹, obtained in fuel celltesting at 0.8 V_(iR-free) vs RHE and 80° C.), optimal mass loss,micropore surface area and surface N content corresponding to theoptimal catalyst from each nominal Fe concentration series are listed inTable 3 (rows 1-3).

TABLE 3 Catalytic activity (in A · g⁻¹, obtained in fuel cell testing at0.8 V_(iR-free) vs RHE and 80° C., under 1.5 bar O₂), optimal mass lossduring pyrolysis, micropore surface area and N content for catalystsproduced at optimal mass loss using various combinations of pyrolysissteps, pore-filler and nominal Fe content. Black Pearls 2000 ™ was usedas the carbon support. The mass ratio of pore-filler to carbon supportwas 1. Catalyst loading for all tests was ca. 1 mg · cm⁻². TheNafion-to-Catalyst ratio (NCR) was 2 unless otherwise noted. NominalOptimal Micropore Pyrolysis Fe Catalytic Mass Surface N Gas and Pore-Content Activity Loss Area Content Temp. Filler (wt %) (A · g⁻¹) (%) (m²· g⁻¹) (at %) 1-Step NH₃ PTCDA 0.2 97 45 716 1.9 Pyrolysis (1050° C.)0.5 106 50 736 1.7 1.0 132 48 710 1.2 Ar Phen 0.2 12 26 188 2 (1050° C.)1.0 24 27 242 2.6 4.1 6 27 330 1.4 2-Step Ar + NH₃ Phen 0.2 260  39* —2.2 Pyrolysis (1050° C.)(950° C.) 1.0 277  35* 497 2.4 4.1 134  33* 3922.4 Ar + NH₃ Phen 1.0 429  47** 580 — (1050° C.)(950° C.) (NCR 1.5:1)*Combined mass loss from two pyrolyses (does not represent optimal massloss). **Combined mass loss from two pyrolyses (represents optimal massloss).Series 3.

The same steps as in 2 were followed for three more series of catalysts,this time produced using Phen as the pore-filler and Black Pearls 2000™as the carbon support with a 50% Phen nominal concentration,corresponding to three different nominal Fe concentrations; 0.2, 1 and4.1 wt %. All pyrolyses were conducted in Ar at 1050° C. The results aresummarized in Table 3 (rows 4-6).

Series 4.

Optimal catalysts from each series (0.2, 1 and 4.1 wt % nominal Feconcentration) in 3. underwent a second pyrolysis, this time in NH3 at950° C. for 5 minutes. The results are summarized in Table 3 (rows 7-9).

The best catalyst from 4. (50% nominal Phen concentration and 1.0 wt %nominal Fe concentration pyrolysed first in Ar at 1050° C. then in NH3at 950° C.) was further optimized by determining the optimal duration ofthe 2nd pyrolysis, or mass loss (ca. 30%) and the optimalNafion-to-catalyst ratio (1.5). The results are summarized in Table 3(row 10).

Conversion from the measured mass activity in A·g−1 under 1.5 bar O₂ tovolumetric activity in A·cm−3 under reference conditions of 1 bar O₂.For comparison to DOE performance targets, the activities of the NPMCsare reported in A·cm⁻³ and under US D.O.E. reference conditions of 1 barO₂ absolute pressure, 100% RH and 80° C. (REFERENCE 26). Themeasurements were performed in reference conditions, except for the O₂pressure (1.5 bar). To convert the mass activity (I_(M)) measured under1.5 bar O₂ absolute pressure to volumetric activity (I_(V)*) under thereference conditions of 1 bar O₂ absolute pressure, Eq. I has beenapplied:

$\begin{matrix}{I_{V}^{*} = {I_{M} \cdot \rho_{eff} \cdot \left( \frac{P_{O\; 2}^{*}}{P_{O\; 2}} \right)^{0.79} \cdot \left( \frac{P_{H\; 2}^{*}}{P_{H\; 2}} \right)^{\alpha_{c}/2}}} & {{Eq}.\mspace{14mu} 1}\end{matrix}$where I_(M) is the mass activity measured under the O₂ and H₂ pressuresP_(O2) and P_(H2) (1.5 bar each), and P_(O2)* and P_(H2)* are thereference pressures (1 bar). A correction must therefore be made toaccount for the difference in O₂ pressure (thermodynamic and kineticcorrection) and H₂ pressure (thermodynamic correction only) betweenreference and actual test conditions. ρ_(eff) is the effective densityof a carbon-based NPMC in the porous cathode. Since it has been shownfor Pt/C catalysts that the kinetic activity at a fixed cell voltage isproportional to P_(O2) ^(0.79) and to P_(H2) ^(αc/2) (where α_(c) is thecathodic transfer coefficient), the same law was assumed to apply tothese NPMC (REFERENCE 48). From FIG. 3, a Tafel slope of 61 mV/dec isfound for the most active NPMC in this study. This slope is nearly equalto that of Pt/C and is similar to most other NPMCs herein. The Tafelslope and α_(c) coefficient are related through Eq. 2:

$\begin{matrix}{{Slope}_{Tafel} = \frac{{R \cdot T \cdot \ln}\; 10}{\alpha_{c} \cdot F}} & (2)\end{matrix}$A value for α_(c) of 1.15 is obtained based on a slope of 61 mV/dec.Theoretically, α_(c) cannot exceed a value of 1. However, assumingα_(c)=1 in Eq. 1, the value of I_(V)* would only increase by ca 3%.Next, ρ_(eff) was previously assumed to be 0.4 g·cm⁻³ (REFERENCE 52).Our direct electrode thickness measurements confirm that the value ofρ_(eff) is indeed very close to 0.4 g·cm⁻³. This value has thereforebeen retained. With regard to porosity, the effective density ofcatalyst of 0.4 g·cm⁻³ corresponds to ca 50% porosity for aNafion-to-NPMC ratio of 1.5. From Eq. 1, it can be seen that multiplyingthe mass activity (in A·g⁻¹; measured under P_(O2) and P_(H2) of 1.5bar, 100% RH and 80° C.) by 0.230 yields the volumetric activity(A·cm⁻³) under the reference conditions (P_(O2) and P_(H2) 1 bar, 100%RH, 80° C.) defined by the U.S. DOE (REFERENCE 26).

In conclusion, the above demonstrates that sufficiently active andinexpensive non-precious metal catalysts for ORR are possible andtherefore offer an alternative to Pt-based catalysts and provide a pathtoward the commercial viability of PEMFCs. These highly active andnear-target-achieving iron-based electrocatalysts have been realizedthanks to an innovative synthesis method based, in this case, on: (i)filling, by planetary ballmilling, the empty pores of a highlymicroporous carbon support (Black Pearls 2000™) with an N-bearingpore-filler (1,10 phenanthroline) and an iron precursor (iron^(II)acetate), and (ii) pyrolysing this catalyst precursor first in Ar at1050° C., then in NH₃ at 950° C.

EXAMPLE 2

Other catalysts according to the present have been prepared by fillingthe micropores of a microporous support by planetary ballmilling withthe following pore-fillers: PTCDA,

The loading of pore-filler ranged from 5 to 85 wt % based on the totalweight of the catalyst.

Using BlackPearl™ as the microporous support, these catalysts showedsatisfying catalytic activity for loading >50 wt %.

Catalysts have also been prepared using different microporous supports:BlackPearls™ (Cabot corp., BET area 1200 m² g−1, surface micropore 900m² g⁻¹), and KetjenBlack™ (Akzo Nobel, BET area 1400 m² g⁻¹, surfacemicropore 500 m² g⁻¹). Pyrolysis was performed in Ar at temperaturesranging from 500 to 1100° C. Interesting catalytic activities have beenobserved at temperatures T>800° C.

Using PTCDA as a pore-filler, both KetjenBlack™ and BlackPearls™ yieldedcatalysts with good activities.

Using KetjenBlack™ as a microporous support and PTCDA, hexacarbonitrileor tetracarbonitrile as a pore-filler also yielded catalysts with goodactivities.

EXAMPLE 3

Catalysts similar to those of Example 1 were prepared with iron acetate,ferrocene and ferricyanide as non-noble metal precursors. Thesecatalysts were prepared using 50 wt % of phenanthroline and 50 wt % ofBlackPearls™ with a nominal Fe concentration of 0.2 wt %. Pyrolysis wasperformed under Ar or NH₃. The same good catalytic activities wereobtained will all non-noble metal precursors.

EXAMPLE 4

This example investigates the influence, on the kinetic activity of thecatalyst, of (a) the type of pore-filler used, (b) the mass ratio ofpore-filler in catalyst precursor, (c) the type of microporous carbonblack used and (d) the nominal iron loading in the catalyst precursor.This expands on the above, where two types of pore-filler and one typeof carbon black were used. Here, four types of pore-filler and two typesof carbon blacks were used (see below). The pore-fillers used are (i)perylene-tetracarboxylic-dianhydride (PTCDA, N-free) (REFERENCE 53),(ii) perylene tetracarboxylic-diimide (PTCDI, N-bearing), (iii) 1,10phenanthroline (phen, N-bearing) that complexes with iron^(II) and has astructure similar to a part of the catalytic site (REFERENCE 31), and(iv) polyacrylonitrile (PAN, N-bearing). The latter has already beenused as a nitrogen precursor by Yeager's group (REFERENCE 6). Themicroporous carbon blacks used are (i) Black Pearls 2000 (BP) from Cabot(BET area 1379 m² g⁻¹, micropore area 934 m² g⁻¹), and (ii) KetjenBlackEC600-JD (KB) from Azko-Nobel (BET area 1405 m² g⁻¹, micropore area 507m² g⁻¹). The mass ratio of pore-filler and the nominal iron loading usedin the catalyst precursors ranged from 0 to 90 wt % and 0.2 to 5 wt %,respectively.

EXPERIMENTAL

Catalyst synthesis. Catalyst precursors were prepared using planetaryballmilling. The iron precursor is iron^(II) acetate (FeAc). Themolecular structures of the various pore-fillers used are:

Planetary ballmilling was performed using ca. 1 g of carbon black andpore-filler mixture (the mass of each depending on the targetedpore-filler mass ratio) and FeAc (the mass depending on the targetednominal iron loading in the catalyst precursor) was placed in a hardenedsteel vial (ca. 65 cm³) together with 20 chrome-steel balls of 0.25 inchdiameter. The ball-to-powder mass ratio was 20:1. The vial was sealed ina nitrogen glove box. Once tightly sealed, the vial was placed in aplanetary ballmiller (FRITSCH Pulverisette 7) to undergo 3 hours ofballmilling at 400 rpm. The resulting powder formed the catalystprecursor. When phen was used as the pore-filler, the phen and iron^(II)(from FeAc) were first complexed by mixing both in a solution of ethanolto form a [Fe(Phen)₃]²⁺ complex. The carbon black was then added to thesolution. This solution was stirred over low heat on a magnetic hotplatefor 2 hours and then placed in a drying oven overnight to be completelydried in air at 90° C. Once dried, the powder was placed in a steel vialand underwent the same ballmilling processing steps as described above.

Two heat-treatment methods for the catalyst precursors were used toobtain active ORR catalysts. In method I, the catalyst precursor waspyrolysed in ammonia at 1050° C. A number of catalysts, each having adifferent mass loss were obtained by pyrolysing samples of the samecatalyst precursor for various pyrolysis times. In Method II, thecatalyst precursor was first pyrolysed in Ar at 1050° C. for 60 minutes.Then, the resulting powder was pyrolysed in NH₃ at 950° C. Here again,for the pyrolysis in NH₃, a number of catalysts, each having a differentmass loss was obtained by pyrolysing samples of the same catalystprecursor for various pyrolysis times. The pyrolysis procedure isdescribed in more detail above.

Electrochemical analysis. The catalyst ink for the fuel cell cathode wasprepared with a Nafion-to-catalyst mass ratio (NCR) of 2 for all samplesexcept for catalysts made with phen where the NCR value was 1.5. Detailsof the ink preparation and MEA assemblies may be found above. The fuelcell tests were performed at 80° C. cell temperature, with thehumidifier for H₂ and O₂ at 105° C. and 95° C. respectively. The backpressures were 15 psig on both sides. The electrode geometric area was1.14 cm². During the initial fuel cell warm-up period, nitrogen was fedto both electrodes. When the fuel cell reached 80° C., nitrogen wasswitched to hydrogen and oxygen at anode and cathode, respectively. Thesystem was then held at OCV for 15 minutes before making an impedancemeasurement and recording the first polarization curve at a potentialscan rate of 0.5 mV/sec using a Parstat 2273 potentiostat. The kineticcurrent at 0.8 V iR-free was estimated by extrapolating the Tafel slopesobserved at higher potential. Conversion from the measured mass activityin A g⁻¹ at 1.5 bar O₂ and 1.5 bar H₂ to volumetric activity in A·cm⁻³at reference conditions of 1 bar is explained in detail above.

Physical analysis. Surface area measurement was performed with aQuantachrome Instruments Autosorb-1 and with N₂ as the adsorbate.Isotherm analysis was performed as in REFERENCE 19. Surface elementalanalysis was performed via X-ray photoelectron spectroscopy using a VGEscalab 200i instrument and the monochromatic source was the Al Kα line(1486.6 eV). Quantification of the elements was performed using Casa XPSsoftware.

Results and Discussion

One-step pyrolysis (method I). The planetary ballmilling method is aneffective method for filling the pores of a carbon black. While pristineBlack Pearls 2000 (BP) has a micropore surface area of 934 m² g⁻¹, afterballmilling of a mixture of 50% BP and 50% PTCDA, the correspondingcatalyst precursor had a micropore surface area of only 65 m² g⁻¹ (FIG.6).

The results shown in FIG. 6 were obtained for samples prepared with BPand PTCDA, with a PTCDA/BP mass ratio of 50/50 and 1 wt % iron nominalloading as FeAc. The mass loss is used as the x-axis variable in FIG. 6.Both the kinetic activity and the micropore surface area reach a maximumat a mass loss of 50% during the NH₃ pyrolysis. FIG. 6 shows that theactivity correlates with the micropore surface area of the catalysts.Moreover, the maximum microporous surface area is ca. 700 m² g⁻¹, i.e.ca. 75% of the original micropore surface area found in pristine BP. Theactivity does not, however, correlate with the N content in thecatalysts. Also, note that 50% mass loss corresponds to the mass ratioof pore-filler in the catalyst precursor. For other ratios ofpore-filler/carbon black it was found that the maximum of activity alsooccurred at a mass loss corresponding to that of the mass ratio of thepore filler in the catalyst precursor. This is understandable since (i)micropores control the activity of such catalysts and (ii) themicropores arise from the etching by NH₃ of the pore-filler in the poresof the microporous carbon black.

For the remainder of this study, for each carbon black/pore-filler/Fenominal loading combination, only the optimized catalyst is reported,i.e. the catalyst for which the mass loss in NH₃-pyrolysis is aboutequal to the pore-filler mass ratio in the catalyst precursor.

FIG. 7A shows the effect of the PTCDA loading in the catalyst precursoron three characteristics of the resulting catalysts: (i) the nitrogencontent (ii) the micropore surface area and (iii) the kinetic current ina PEM fuel cell. These catalysts were prepared using method I and thenominal iron loading in the catalyst precursors was fixed at 0.2 wt %.The ORR activity in fuel cell appears to follow the same trend asnitrogen content with PTCDA loading, except for a PTCDA loading of 90%,where we observe a sudden decrease in nitrogen content, but no decreasein activity. The micropore surface area is roughly equal for all PTCDAloadings and corresponds in part to micropores created during pyrolysisdue to the etching of pore-filler by ammonia. These observations mightlead to the conclusion that the N content limits the activity. However,an alternative explanation is also possible. Although the nominal ironloading was fixed at 0.2 wt %, the final iron content in the catalystsof FIG. 7A changes: it may be estimated as 0.2 wt %·100/(100−X), where Xis the PTCDA mass ratio in wt %. This calculation is based on theassumption that all iron content present in the catalyst precursorremains in the catalyst, while the PTCDA content is etched by NH₃. Theretention of Fe during pyrolysis has been shown for a series of priorart catalysts (REFERENCE 21). For example, the catalyst in FIG. 7A madeusing 75 wt % PTCDA in the catalyst precursor contains ca. 0.8 wt % Fewhile the one made with 20 wt % PTCDA contains ca. 0.25 wt % Fe. Anincrease of catalytic activity by increasing iron loading up to 1 wt %Fe will be shown below when discussing FIG. 7B. The activity of theseries of catalysts in FIG. 7A can therefore be limited by either the Ncontent or the Fe content or a combination of both.

In conclusion, for a nominal iron loading of 0.2 wt % (FIG. 7A), thehighest kinetic activity was 116 A·g⁻¹ at 0.8V iR-free and was obtainedfor a PTCDA loading of 75%. The optimal mass loss for this catalyst was75%, i.e. the mass ratio of PTCDA in the catalyst precursor.

Next, with a fixed PTCDA mass ratio of 50% in the catalyst precursor,the nominal iron loading was varied from 0.2 to 5 wt %. For all nominaliron loadings, pyrolysis in NH₃ at 1050° C. were repeated until ca. 50wt % mass loss was obtained; a value corresponding to the mass ratio ofPTCDA in the catalyst precursor. Consequently, the micropore surfacearea for all these catalysts (see FIG. 7B, upper graph, stars) arealmost exactly the same. The time of pyrolysis required to obtain 50%mass loss increased with increasing nominal iron loading. This isbelieved to be due to a decreasing reaction rate between PTCDA and NH₃with increasing iron content (REFERENCE 54), and may be attributed tothe competing decomposition reaction of NH₃ into N₂ and H₂ in thepresence of excess iron (in the form of aggregates), as it is known tobe an effective catalysts for such a reaction (REFERENCE 55).

The upper graph in FIG. 7B shows that the nitrogen content in thecatalysts (square) decreases with nominal iron loading up to ca. 1 wt %and remains constant beyond this value. The decrease in N content withincrease in nominal iron loading may be a result of the competingdecomposition reaction of NH₃ described above. The ORR kinetic activityin fuel cell is shown in the lower graph in FIG. 7B. It increases withincreasing iron loading up to 1 wt %, then decreases gradually withincreasing nominal iron loading beyond 1 wt %.

Thus, the kinetic activity of catalysts with increasing nominal ironloading up to 1 wt % appears to be limited by iron content, while thelow kinetic activity of catalysts with nominal iron loadings >1 wt %seem to be limited by low N content.

For the catalysts shown in FIG. 7B, the maximum kinetic activityobtained was 132 A·g⁻¹ at 0.8V iR-free and corresponds to a nominal Feloading of 1 wt %. This maximum kinetic activity is believed to be theresult of an optimal balance between Fe and N content.

Two-steps pyrolysis (method II). The results obtained with method II(first pyrolysis in Ar, second pyrolysis in NH₃) are shown in FIG. 8.The pore-filler used in this case is phen, the carbon black and ironprecursor are still BP and FeAc, respectively. FIG. 8 is laid outsimilarly to FIG. 7, except that phen is used as the pore-filler andmethod II was used for the pyrolysis steps (see experimental).

FIG. 8A shows how N content, microporous surface area and ORR kineticactivity in fuel cell varies with the phen mass ratio in the catalystprecursor. The iron loading was fixed at 1 wt %. As the phen mass ratioincreases, the micropore surface area continually decreases while thenitrogen content initially increases (up to ca. 50 wt % phen) thenlevels-off. Unlike the catalysts presented in FIG. 7, where the sole Nprecursor is NH₃, here two N precursors are introduced, namely phen andNH₃. The ORR activity in fuel cell seems to be influenced by both the Ncontent and the micropore surface area. The maximum kinetic activity forcatalysts made with phen as the pore-filler was 429 A·g⁻¹ at 0.8ViR-free using 50 wt % phen mass ratio (FIG. 8A, lower graph). The ironcontent of this catalyst is ca. 2 wt % (based on 1 wt % nominal ironloading and ca. 50 wt % mass loss during pyrolysis).

Next, the phen mass ratio was fixed at 50 wt % and the iron loading wasvaried from 0.2 to 5 wt % (FIG. 8B). The micropore surface area wasroughly the same for different nominal iron loadings, but the N contentbehaves differently with a maximum of ca. 2.4 at % for a Fe loading of 1wt %. The kinetic activity for these catalysts follows the same trend asthe N content. If one were to plot the activity vs. N content, a linearrelation would be observed.

Overall, for all catalysts presented in FIG. 8, the optimum Fe loadingwas 1 wt % for a 50 wt % phen mass ratio corresponding to a maximumkinetic activity of 429 A·g⁻¹ at 0.8V iR-free. This activity is higherthan the maximum activity for catalysts presented in FIG. 7 (132 A·g⁻¹at 0.8V iR-free).

Effect of pore-filler used with method I (one-step pyrolysis). Catalystsmade with three different pore-fillers (PTCDA, PTCDI and PAN) and heattreated in NH₃ at 1050° C. were investigated. The pore-filler mass ratiowas kept constant at 50 wt % and the nominal iron loading was fixed at 1wt %. Pyrolysis were repeated until 50 wt % mass loss was obtained forcatalysts made with each pore-filler.

The first three rows in Table 4 represent the results for catalysts madewith PTCDA, PTCDI and PAN, respectively. The nitrogen content andmicroporous surface areas of these catalysts are similar. The kineticactivity for the catalyst made with PTCDA is about twice as high as thatmade with PTCDI or PAN. For the latter, the apparent Tafel slope ishigher than that of either the PTCDA- or the PTCDI-based catalyst (insetof FIG. 9). This results in a low kinetic activity at 0.8V iR-free forthe PAN-based catalyst (Table 4). Thus, the comparison of the kineticactivities should preferably be done at 0.9V rather than at 0.8V.

Effect of microporous carbon black used with method I (one-steppyrolysis). Catalysts made with two different carbon blacks (BP and KB),but similar methods were investigated. The pore-filler and mass ratioused for both catalysts was PTCDA and 50 wt %, respectively. The ironloading was 1 wt %. The catalyst precursor prepared by planetaryballmilling was pyrolysed in NH₃ at 1050° C. Pyrolysis were repeateduntil 50 wt % mass loss was obtained for both catalysts. Rows 1 and 4 inTable 4 represent these two catalysts, where the only difference is themicroporous carbon black used. The kinetic activity obtained with thecatalyst using KB+PTCDA is lower than that obtained with BP+PTCDA, allother things equal. This lower kinetic activity is consistent with thelower microporous surface area and N content in the (KB+PTCDA)-basedcatalyst.

Activity and mass-transport: a necessary trade-off? While high kineticactivity at 0.8 or 0.9V iR-free is desirable, so is high power densityat reasonably high cell voltage. Thus, good mass transport properties ofelectrodes made with these catalysts is also desirable.

FIG. 9 presents the fuel cell polarization curves recorded using O₂/H₂for all catalysts listed in Table 4. The catalyst loading was 1 mg·cm⁻².All catalysts were synthesized according to method I (one pyrolysis inNH₃ at 1050° C.) except for the catalyst BP+phen which was synthesizedaccording to method II (first pyrolysis in Ar and the second in NH₃).FIG. 9 shows that the performance in fuel cell at 0.5 V iR-free does notnecessarily correlate with the kinetic activity of the catalysts,measured at 0.8 or 0.9 V iR-free. The higher kinetic activity of theBP+phen catalyst does not translate into better performance at 0.5V. TheKB+PTCDA catalyst, for example, has much lower activity at 0.8V or 0.9ViR-free, but better performance than the BP+phen catalyst at 0.5ViR-free.

TABLE 4 Nitrogen content, microporous surface area, and kinetic activityat 0.9 V (in A g⁻¹) and 0.8 V iR-free (in A g⁻¹ and A cm⁻³), forcatalysts made with various combinations of carbon black/pore- filler.In all cases the pore-filler mass ratio and nominal iron content was 50%and 1 wt %, respectively. Current Current Current Catalyst NitrogenMicropores at 0.9 V at 0.8 V at 0.8 V precursor at % m² g⁻¹ A g⁻¹ A g⁻¹A cm⁻³ BP + PTCDA 1.2 710 2.6 132 30 BP + PTCDI 0.7 802 1.2 76 17 BP +PAN 1.1 821 1.2 36 8 KB + PTCDA 0.6 582 0.9 37 8 BP + Phen 2.4 580 6.8429 99

Although the present invention has been described hereinabove by way ofspecific embodiments thereof, it can be modified, without departing fromthe spirit and nature of the subject invention as defined in theappended claims.

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The invention claimed is:
 1. A catalyst comprising: a carbon-basedhighly microporous support, and a high density of catalytic siteslocated in the micropores of the microporous support, so that thecatalyst has a volumetric activity for oxygen reduction reaction ofabout 22 A·cm³ or more, as measured at 0.8 V iR-free cell voltage,wherein the graphite sites are in electronic contact with the walls ofthe micropores of support, and wherein the catalytic sites comprise acarbon poly-aromatic structure extending from graphite platelets on theedge of graphitic crystallites within the micropores, at least onenon-noble metal atom, and at least about four nitrogen atoms, whereinthe catalyst has been prepared by pyrolysing a catalyst precursor,wherein, before pyrolysis, the catalyst precursor comprises saidmicroporous support with, micropores filled by a pore-filler and anon-noble metal precursor so that the micropore surface area of thecatalyst precursor is substantially smaller than the micropore surfacearea of the support when the pore-filler and the non-noble metalprecursor are absent, wherein the pyrolysis is performed in the presenceof a gas that is a nitrogen precursor when the microporous support, thenon-noble metal precursor and the pore-filler are not nitrogenprecursors, wherein, during pyrolsis, the pore-filler and the non-noblemetal precursor react, thereby creating said catalytic sites within saidmicropores, and wherein, after pyrolysis, the micropore surface area ofthe catalyst is substantially larger than the micropore surface area ofcatalyst precursor.
 2. The catalyst of claim 1, wherein at least one ofthe microporous support, the non-noble metal precursor, or thepore-filler is a nitrogen precursor.
 3. The catalyst of claim 1, whereinthe highly microporous support has a microporous surface area of about500 m²/g more.
 4. The catalyst of claim 1, wherein the non-noble metalis iron or cobalt.
 5. The catalyst of claim 4 having an iron loading ofabout 0.2 wt % or more based on the total weight of the catalystprecursor.
 6. The catalyst of claim 1, wherein the non-noble metalprecursor is a salt of a non-noble metal or an organometallic complex ofa non-noble metal.
 7. The catalyst of claim 1, wherein the non-noblemetal precursor and the pore-filler are the same materials.
 8. Thecatalyst of claim 1, wherein the pore-filler comprises a polycyclicstructure.
 9. The catalyst of claim 1, wherein the micropore surfacearea of the catalyst precursor is less than about 50% of the microporesurface area of the support when the pore-filler and the non-noble metalprecursor are absent.
 10. The catalyst of claim 1, wherein the microporesurface area of the catalyst is more than about 50% of the microporesurface area of the support when the pore-filler and the non-noble metalprecursor are absent.
 11. The catalyst of claim 1, wherein a mass lossduring pyrolysis is about equal to a pore-filler loading in the catalystprecursor.
 12. The catalyst of claim 1 having a nitrogen content ofabout 0.5 wt % or more based on the total weight of the catalyst. 13.The catalyst of claim 1, wherein the microporous support is a carbonblack or an activated carbon, the non-noble metal precursor is Fe^(II)acetate, and the pore-filler is perylene -tetracarboxylic-dianhydride,1,10-phenanthroline, perylene tetracarboxylic-diimide, orpolyacrylonitrile.
 14. The catalyst of claim 1, where the volumetricactivity for oxygen reduction reaction is about 60 A·cm³ or more, asmeasured at 0.8 V iR-free cell voltage.
 15. A method of producing thecatalyst of claim 1, the method comprising a. providing a catalystprecursor as defined in claim 1; and b. pyrolyzing said catalystprecursor, thereby producing a catalyst with a micropore surface areasubstantially larger than the micropore surface area of the catalystprecursor, wherein the pyrolysis is performed in the presence of a gasthat is a nitrogen precursor when the microporous support, the non-noblemetal precursor and the pore-filler are not nitrogen precursors.
 16. Themethod of claim 15, wherein said providing in step a) comprises: 1.providing a microporous support; a non-noble metal precursor; and apore-filler; and
 2. filling the micropores of the microporous supportwith the pore-filler and the non-noble metal precursor so that themicropore surface area of the catalyst precursor is substantiallysmaller than the micropore surface area of the support when thepore-filler and the non-noble metal precursor are absent.
 17. The methodof claim 16, wherein the micropores of the microporous support arefilled with the pore-filler and the non-noble metal precursor byballmilling or resonant acoustic mixing with or without a grindingmedium.
 18. The method of claim 15, wherein the pyrolysis is performedin a nitrogen-containing reactive gas or vapor.
 19. The method of claim15, wherein the pyrolysis is performed in an inert gas.
 20. The methodof claim 19, wherein a second pyrolysis in a nitrogen-containingreactive gas or vapor is performed following the pyrolysis performed inthe inert gas.